Families I've only just met who insist on explaining the intricacies of their blended families. "Well, Billy is my partner's son by his first marriage, and Suzie is mine by my second marriage, and Bobby is all ours". I DON'T CARE. Just say "These are our children" and leave it at that. I'll probably never see you again...at least, I hope not.
Along the same lines, I hate people who ask personal questions about a child's parentage. I have personal reasons for hating this. First of all, my Dutch, blue-eyed, fair-skinned parents adopted my two older brothers, who have Greek heritage. None of us will ever forget the horrible woman who asked my mother point-blank where she got the two darker kids from. Also, my two oldest children are 12 months apart, and look nothing alike. When they were young and I was pushing them around in a double pushchair I had people ask me if they were both mine. The ultimate over-the-line question was whether they both had the same father. I pointed at one of the kids and said perfectly straight-faced that this child was the milkman's. I understand that people may be curious, but asking such unsolicited questions is so bloody rude.